The composition of coal as mined varies considerably, in particular with respect to the amount of ash-forming constituents and water. Certain types of brown coal may contain up to 70 percent by weight of water, largely bound chemically. The content of ash-forming constituents may amount to 40 percent by weight. The term brown coal is used here to indicate a variety of coals outside the range of hard coals and includes sub-bituminous coal, lignite and unconsolidated brown coal. Other materials belonging to the class defined above are peat, wood, paper, vegetable material, sewage sludge, etc.
It is highly desirable to upgrade such materials in order to improve their performance in various applications, such as combustion, or gasification processes. Moreover, upgrading will result in a considerable reduction in transport costs.
It is known that at high temperature coal not only loses chemically bound water, but undergoes such a change that at least no complete reabsorption of water will occur, not even when the coal is kept in a water phase at high pressure. This is due to a change in the coal itself, a phenomenon known as coalification. Application of a pressure exceeding the vapor pressure of water prevents the evaporation of the freed water, thus reducing the costs of the dewatering process. In view of the desirability of utilizing as an energy source the huge quantities of coal still available, in particular the softer grades, it should be possible to upgrade the coal in large quantities in continuous operation.
Attempts have been made to dewater brown coal and similar material by subjecting it to a heat treatment at elevated pressure. The philosophy underlying these attempts was that a charge of brown coal to be heat-treated should contain as little water as possible to begin with, since obviously a minimum of water then needs to be separated from it during and after the heat treatment.
Accordingly, it has been proposed in the prior art to pressurize a load of brown coal that has a dry appearance and subsequently to heat this load, whereafter any adherent water and the water that has been driven out thermally is separated, e.g., mechanically, from the brown coal so dewatered. Separation of the water is carried out at the earliest stage possible in the process, i.e. before depressurizing the brown coal.
A process based on the above principles would be rather complicated since it involves pressurizing a stream of solid material, heating the stream of pressurized solid material, separating brown coal and water mechanically at high pressure, depressurizing both the stream of dewatered brown coal and the stream of water and cooling either the combined or the separated streams of dewatered brown coal and water.